Prescription eyeglass lenses are curved in such a way that light is correctly focused onto the retina of a patient's eye, improving vision. Such lenses are formed from glass or plastic lens “blanks” having certain desired properties to provide the correct prescription for the patient. The blanks are usually circular and of substantially larger dimension compared to the relatively smaller finished lenses assembled into eyeglass frames. Therefore, a lens blank must be edged to fit an eyeglass frame selected by the patient.
Ophthalmic laboratory technicians cut, grind, edge, and polish blanks according to prescriptions provided by dispensing opticians, optometrists, or ophthalmologists. The specifications include the patient's full prescription, including: 1) the total power the finished lens must have; 2) the strength and size of any segments, if needed (i.e. multifocal lenses); 3) the power and orientation of any cylinder curves; and 4) the location of the optical center and any inducted prism that may be needed.
In addition, the large diameter blank is sized and shaped to fit into the frame selected by the patient. The lens blank may be shaped using an edger, such as the edger disclosed in U.S. Pat. No. 6,203,409 to Kennedy et al., the disclosure of which is incorporated herein by reference. The blank is edged so that the periphery of the finished lenses fit into the openings on the frames.
Edging of a lens blank typically requires the application of a block to a surface thereof. The block is releasably secured to a clamp assembly, so that rotation of the clamp assembly causes corresponding rotation of the lens blank. As the blank is rotated, the periphery of the blank may be cut to a desired size using a router tool. The blank may be either ground or cut. Wet edgers use diamond-impregnated wheels with different abrasive grits to grind the lens material. A coolant is sprayed on the wheels during edging to reduce heat. Dry edgers use carbide steel or diamond blades mounted on the spindle of a motor to shave the lens. The lens periphery may also be polished using a polishing tool. some edgers are also able to form a groove about the periphery of the lens.
The finished lens may then be assembled with the selected eyeglass frames. Many frames have a bevel extending around the inner circumference of the openings. The bevel interlocks with a complementarily shaped groove formed about the peripheral edge of the lens. The interlock between the complementary bevel and groove helps to secure the lens within the frame opening.
In order to improve efficiency, some edgers use CNC (Computer Numeric Control) technology whereby a computer controls the lens processing equipment by following encoded commands. The commands are based on information from frame tracings or internal lens probes and the user. Information relating to the size and shape of the lens needed for a particular frame (i.e. trace data) may be generated, and subsequently transmitted to the edger. The trace data may be stored in a control system, such as a central processing unit, in communication with the edger.
Some lenses require that the lens have a groove in the peripheral edge of the lens, and also contain drill features in the surface of the lens. For example, some frame assemblies require that one or more holes be drilled in the lenses, particularly lenses to be used in rimless style frames. Several factors to consider when determining the hole position include the horizontal and vertical coordinates, lens base curve, wrap angle, and the mounting's pantoscopic tilt. Hand drilling is used by some laboratories. Other laboratories use a drill press.
Conventional drilling devices include a computer program and control system separate from the control system for edging and grooving the blank. Thus, a lens blank must first be edged and grooved, and then separately drilled to produce the desired lens, thereby increasing manufacturing time and cost.